diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 2ca1e70..87d6df6 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -4,10 +4,10 @@
 from itertools import chain
 from test import test_support
 
-import io
+import io  # The module under test
 
 
-class MockIO(io.RawIOBase):
+class MockRawIO(io.RawIOBase):
 
     def __init__(self, read_stack=()):
         self._read_stack = list(read_stack)
@@ -56,13 +56,13 @@
 
 class MockNonBlockWriterIO(io.RawIOBase):
 
-    def __init__(self, blockingScript):
-        self.bs = list(blockingScript)
+    def __init__(self, blocking_script):
+        self._blocking_script = list(blocking_script)
         self._write_stack = []
 
     def write(self, b):
         self._write_stack.append(b[:])
-        n = self.bs.pop(0)
+        n = self._blocking_script.pop(0)
         if (n < 0):
             raise io.BlockingIOError(0, "test blocking", -n)
         else:
@@ -90,6 +90,23 @@
         f.seek(-2, 2)
         f.truncate()
 
+    def large_file_ops(self, f):
+        assert f.readable()
+        assert f.writable()
+        self.assertEqual(f.seek(2**32), 2**32)
+        self.assertEqual(f.tell(), 2**32)
+        self.assertEqual(f.write(b"xxx"), 3)
+        self.assertEqual(f.tell(), 2**32 + 3)
+        self.assertEqual(f.seek(-1, 1), 2**32 + 2)
+        f.truncate()
+        self.assertEqual(f.tell(), 2**32 + 2)
+        self.assertEqual(f.seek(0, 2), 2**32 + 2)
+        f.truncate(2**32 + 1)
+        self.assertEqual(f.tell(), 2**32 + 1)
+        self.assertEqual(f.seek(0, 2), 2**32 + 1)
+        self.assertEqual(f.seek(-1, 2), 2**32)
+        self.assertEqual(f.read(2), b"x")
+
     def read_ops(self, f):
         data = f.read(5)
         self.assertEqual(data, b"hello")
@@ -130,19 +147,9 @@
         f = io.BytesIO(data)
         self.read_ops(f)
 
-    def test_fileio_FileIO(self):
-        import _fileio
-        f = _fileio._FileIO(test_support.TESTFN, "w")
-        self.assertEqual(f.readable(), False)
-        self.assertEqual(f.writable(), True)
-        self.assertEqual(f.seekable(), True)
-        self.write_ops(f)
-        f.close()
-        f = _fileio._FileIO(test_support.TESTFN, "r")
-        self.assertEqual(f.readable(), True)
-        self.assertEqual(f.writable(), False)
-        self.assertEqual(f.seekable(), True)
-        self.read_ops(f)
+    def test_large_file_ops(self):
+        f = io.open(test_support.TESTFN, "w+b", buffering=0)
+        self.large_file_ops(f)
         f.close()
 
 
@@ -205,7 +212,7 @@
 class BufferedReaderTest(unittest.TestCase):
 
     def testRead(self):
-        rawio = MockIO((b"abc", b"d", b"efg"))
+        rawio = MockRawIO((b"abc", b"d", b"efg"))
         bufio = io.BufferedReader(rawio)
 
         self.assertEquals(b"abcdef", bufio.read(6))
@@ -231,7 +238,7 @@
 
     def testReadNonBlocking(self):
         # Inject some None's in there to simulate EWOULDBLOCK
-        rawio = MockIO((b"abc", b"d", None, b"efg", None, None))
+        rawio = MockRawIO((b"abc", b"d", None, b"efg", None, None))
         bufio = io.BufferedReader(rawio)
 
         self.assertEquals(b"abcd", bufio.read(6))
@@ -241,19 +248,19 @@
         self.assertEquals(b"", bufio.read())
 
     def testReadToEof(self):
-        rawio = MockIO((b"abc", b"d", b"efg"))
+        rawio = MockRawIO((b"abc", b"d", b"efg"))
         bufio = io.BufferedReader(rawio)
 
         self.assertEquals(b"abcdefg", bufio.read(9000))
 
     def testReadNoArgs(self):
-        rawio = MockIO((b"abc", b"d", b"efg"))
+        rawio = MockRawIO((b"abc", b"d", b"efg"))
         bufio = io.BufferedReader(rawio)
 
         self.assertEquals(b"abcdefg", bufio.read())
 
     def testFileno(self):
-        rawio = MockIO((b"abc", b"d", b"efg"))
+        rawio = MockRawIO((b"abc", b"d", b"efg"))
         bufio = io.BufferedReader(rawio)
 
         self.assertEquals(42, bufio.fileno())
@@ -268,7 +275,7 @@
 
     def testWrite(self):
         # Write to the buffered IO but don't overflow the buffer.
-        writer = MockIO()
+        writer = MockRawIO()
         bufio = io.BufferedWriter(writer, 8)
 
         bufio.write(b"abc")
@@ -276,7 +283,7 @@
         self.assertFalse(writer._write_stack)
 
     def testWriteOverflow(self):
-        writer = MockIO()
+        writer = MockRawIO()
         bufio = io.BufferedWriter(writer, 8)
 
         bufio.write(b"abc")
@@ -305,13 +312,13 @@
         # later.
 
     def testFileno(self):
-        rawio = MockIO((b"abc", b"d", b"efg"))
+        rawio = MockRawIO((b"abc", b"d", b"efg"))
         bufio = io.BufferedWriter(rawio)
 
         self.assertEquals(42, bufio.fileno())
 
     def testFlush(self):
-        writer = MockIO()
+        writer = MockRawIO()
         bufio = io.BufferedWriter(writer, 8)
 
         bufio.write(b"abc")
@@ -323,8 +330,8 @@
 class BufferedRWPairTest(unittest.TestCase):
 
     def testRWPair(self):
-        r = MockIO(())
-        w = MockIO()
+        r = MockRawIO(())
+        w = MockRawIO()
         pair = io.BufferedRWPair(r, w)
 
         # XXX need implementation
@@ -333,7 +340,7 @@
 class BufferedRandomTest(unittest.TestCase):
 
     def testReadAndWrite(self):
-        raw = MockIO((b"asdf", b"ghjk"))
+        raw = MockRawIO((b"asdf", b"ghjk"))
         rw = io.BufferedRandom(raw, 8, 12)
 
         self.assertEqual(b"as", rw.read(2))